oeZ WALTER H. BUCHER 



Horizontal faults "occur by the score in the shale beds," 

 usually offsetting the vertical joints for distances often measuring 

 several inches. 



These "faults" are unquestionably shearing planes forming an 

 acute angle of approximately 60° facing the direction of the hori- 

 zontal compressive stress which is clearly manifested in the 

 differential movement between adjoining layers of shale referred 

 to above as "horizontal faults." 



Identical results have been obtained in the laboratory, when- 

 ever sufficiently brittle materials were subjected to horizontal 

 compression in the course of experiments on overthrusting,^ and 

 before our eye loom up the sections of the mountain ranges which 

 these experiments were to help explain, with thrust faults in 

 astonishing numbers and on gigantic scales. 



Here the subject of our discussion assumes different proportions. 

 We realize that the grouping of stresses resulting in the formation 

 of any of the fracture systems discussed before remains the same 

 whether the fracturing finally results in the formation of vast 

 lines of displacement generally referred to as major faults, which 

 may bound mountain ranges or even continents, or ends with the 

 production of minute cracks which, filled with white calcite, form 

 a delicate network on the dark rock and, found on the surface 

 of flat pebbles on the wet beach, are the delight of children. 



Before we can extend the application of Hartmann's law to 

 the larger scale of the great overthrusts of folded mountains, we 

 must first answer a question which now assumes fundamental 

 importance : Is the angle of shear, in a given substance, sufficiently 

 constant under widely different conditions of pressure and tem- 

 perature so as to exclude the possibility of grave errors in its use ? 



We may approach this question best by turning to the ingenious 

 mathematical theory which Mohr has given to account for the 

 results of Hartmann's and others' experiments. 



' See, for instance, H. M. Cadell, "Experimental Researches in Mountain Build- 

 ing," Trans, Roy. Soc. Edinburgh, Vol. XXXV (1890), pp. 337-57; and R. T. Cham- 

 berlin and W. Z. Miller, "Low-Angle Faulting," Jour. Geol., Vol. XXVI -{fq 18), 

 pp. 1-44, especially Fig. 9. 



Note, however, that the position of the strain ellipsoids in the rigid layers in 

 Fig. 10 does not correspond to the writer's interpretation. 



[To be continued] 



